The invention relates to a nitrile rubber, a process for producing it, vulcanizable mixtures based on this nitrile rubber, also a process for producing vulcanizates from these mixtures and the vulcanizates obtained in this way.
For the purposes of the present invention, nitrile rubbers, also referred to as “NBRs” for short, are rubbers which are copolymers or terpolymers of at least one α,β-unsaturated nitrile, at least one conjugated diene and optionally one or more further copolymerizable monomers.
Such nitrile rubbers and processes for producing such nitrile rubbers are known, see, for example, W. Hofmann, Rubber Chem. Technol. 36 (1963) 1 and Ullmann's Encyclopedia of Industrial Chemistry, VCH Verlagsgesellschaft, Weinheim, 1993, pp. 255-261.
Nitrile rubbers are used across a very wide variety of fields of application, as for example for seals, hoses, valve seals and damping elements in the automotive sector, and also for hoses, stators and borehole seals in the oil extraction field, and also for numerous components in the aeronautical industry, the electrical industry, mechanical engineering and marine engineering. For different forms of use it is important that the nitrile rubbers have no deleterious effects—such as corrosion, for example—on the components with which they come into contact. This applies particularly to those rubber parts which are in contact or come into contact on the one hand with water, water-containing solvents and fuels, and also water vapour, and, on the other hand, with metals or metal-containing components. Such parts are, in particular, seals, hoses and diaphragms. Corresponding rubber articles are as follows: O-rings and flat seals, cooler hoses, servo control hoses, air conditioner hoses, thermal insulation hoses, and diaphragms for hydro bearings or diaphragm pumps, for example.
NBR is produced by emulsion polymerization, which firstly gives an NBR latex. The NBR solid is isolated from this latex by coagulation. There is a very wide range of variants for carrying out this coagulation. Salts and acids are conventionally used for coagulation. The stated aim is usually to keep the electrolyte amounts as low as possible.
In the coagulation of latices by means of metal salts, it is known that significantly larger amounts of electrolyte are required in the case of monovalent metal ions, e.g. in the form of sodium chloride, than in the case of polyvalent metal ions, e.g. in the form of calcium chloride, magnesium chloride or aluminium sulphate (Kolloid-Z. 54, 154 (1957)). It is also known that the use of polyvalent metal ions leads to “at least some inclusion of the emulsifier in the product” (Houben-Weyl (1961), Methoden der Org. Chemie, Makromolekulare Stoffe 1, p. 484). According to Houben-Weyl (1961), Methoden der Org. Chemie, Makromolekulare Stoffe 1, p. 479, “not only do the electrolytes used have to be very carefully washed out again, but the finished product should also be free of the catalysts and emulsifiers of the process batch. Even small amounts of residual electrolytes give turbid and cloudy pressed and injection-moulded parts, impair the electrical properties and increase the water absorption capacity of the finished product” (citation). More precise details of the type and amount of impurities of such nitrile rubbers and the effect thereof on the properties of nitrile rubber moldings in contact with other components are not given.
DD 154 702 discloses a process for the free-radical copolymerization of butadiene and acrylonitrile in emulsion, which is controlled by means of a specific, advantageously computer-aided metering program for the monomers and the molecular weight regulators, e.g. tert-dodecyl mercaptan, and in which the latices obtained are worked up by coagulation in an acid medium to give the solid rubber. A significant advantage of the process is said to be that the resin soaps and/or fatty acid soaps used as emulsifiers remain in the rubber as a result of the use of acids in the coagulation, i.e. they are not washed out as in the case of other processes. In addition to the advantage of good properties of the NBR, the improvement in the economics of the process and the avoidance of wastewater pollution by washed-out emulsifier are specifically advertised here. It is stated that the butadiene-acrylonitrile copolymers containing 10-30% by weight of acrylonitrile obtained have good elasticity and low-temperature properties combined with an increased swelling resistance and advantageous processability. No details are given about the type and amount of impurities of these nitrile rubbers. In DD 154 702 these are also no details about the metal corrosivity of the vulcanizates produced with these nitrile rubbers.
According to JP 27902/73 (Appl. 69 32,322) the coagulation is carried out with magnesium salts in the presence of amines. The combination of diethylenetriamine and magnesium chloride is used, for example.
DE-A 23 32 096 discloses that rubbers can be precipitated from their aqueous dispersions by means of methylcellulose and a water-soluble alkali metal, alkaline earth metal, aluminium or zinc salt. Preference is given to using sodium chloride as water-soluble salt. It is stated that an advantage of this process is that it gives a coagulum which is virtually completely free of extraneous constituents such as emulsifiers, catalysts residues and the like since these extraneous materials are removed together with the water when the coagulum is separated off and any remaining residues are completely washed out by means of further water. In DE-A 24 25 441, the electrolyte coagulation of rubber latices is carried out using 0.1-10% by weight (based on the rubber) of water-soluble C2-C4 alkylcelluloses or hydroxyalkylcelluloses in combination with from 0.02 to 10% by weight (based on the rubber) of a water-soluble alkali metal, alkaline earth metal, aluminium or zinc salt as auxiliary instead of methylcellulose. Here too, preference is given to using sodium chloride as water-soluble salt. The coagulum is separated off mechanically, optionally washed with water and the remaining water is removed. Here too, it is stated that the extraneous materials are, as in DE-A 23 32 096, essentially completely removed together with the water when the coagulum is separated off and any remaining residues are washed out completely in the washing with further water. No details are given about the residual amounts of the impurities in these nitrile rubbers. Furthermore, neither DE-A 23 32 096 nor DE-A 24 25 441 gives details about the effects of some impurities on the property of the metal corrosivity of the vulcanizates produced with these nitrile rubbers.
U.S. Pat. No. 5,708,132 describes a process for working up nitrile rubber latices, which displays improved storage stability (70° C./28 days) and a higher full vulcanization rate (TC90). Mixtures of salts and acids, in particular sulphuric acid, are used for coagulation of the latex. The process is characterized by maintenance of a narrow pH range in the washing of the crumb, with the pH of the washing water being in the range from 5 to 8, preferably from 5.5 to 7.5, particularly preferably from 6 to 7. Calcium hydroxide, magnesium hydroxide and sodium hydroxide are used for adjusting the pH, with the use of sodium hydroxide being preferred. An ageing inhibitor based on alkylated aryl phosphites, in particular alkylated aryl phosphites in combination with sterically hindered phenols, is used for stabilizing the nitrile rubber. After washing, the rubber crumb is dewatered in a screw apparatus to residual moisture contents of from 7 to 10% by weight and subsequently dried thermally.
In DE-A 27 51 786, it is established that the precipitation and isolation of rubbers from their aqueous dispersions can be carried out by means of a smaller amount of (hydroxy)alkylcellulose when from 0.02 to 0.25% by weight of a water-soluble calcium salt is used. A further advantage is said to be that this process gives an extremely pure coagulum which is essentially completely free of extraneous constituents such as emulsifiers, catalysts residues and the like. These extraneous materials are removed together with the water when the coagulum is separated off and any remaining residues can be washed out by means of water. It is also stated that the properties of the isolated rubbers are not adversely affected by a calcium salt being used for coagulation. Rather, it is said that a rubber whose vulcanizate properties are not impaired and are fully satisfactory is obtained. This is presented as surprising since it is said that impairment of the rubber properties is frequently observed when polymers are precipitated from dispersions by means of polyvalent metal ions such as calcium or aluminium ions. Houben-Weyl (1961), Methoden der Org. Chemie, Makromolekulare Stoffe 1, pp. 484/485, is offered as evidence for the last statement. In contrast, the rubbers of DE-A 27 51 786 display no slowing or worsening of, for example, the initial vulcanization and/or full vulcanization. No details are given about the residual amounts of the impurities in these nitrile rubbers. DE-A 27 51 786 likewise does not give any information about the possible effects of such impurities.
As in the case of the above-described patents, the object of DE-A 30 43 688, is also to achieve a large reduction in the amounts of electrolyte required for coagulation of the latex. According to the teachings of DE-A 3043 688, this is achieved by using either plant-based protein-like materials or polysaccharides such as starch and if appropriate water-soluble polyamine compounds as auxiliaries in addition to the inorganic coagulate in the electrolyte coagulation of latices. As inorganic coagulates, preference is given to alkali metal or alkaline earth metal salts. The specific additives make it possible to achieve a reduction in the amounts of salts used for quantitative coagulation of the latex. No details are given about the residual amounts of the impurities in these nitrile rubbers. Furthermore, there are no details about the effects of such impurities in vulcanizates based on these nitrile rubbers.
According to U.S. Pat. No. 2,487,263, the coagulation of the latex of styrene-butadiene rubbers is not carried out using metal salts but by means of a combination of sulphuric acid with gelatin (“glue”). The amount and concentration of the sulphuric acid are selected so that the pH of the aqueous medium is set to a value of <6. It is stated that it is advantageous for discrete rubber crumbs which are not coherent and can readily be filtered off and can readily be washed to be formed in the coagulation of the latex. Styrene-butadiene rubber obtained according to the teaching of U.S. Pat. No. 2,487,263 has a lower water absorption capacity, a lower ash content and a higher electrical resistance than rubbers coagulated by means of salts without the addition of gelatin. U.S. Pat. No. 2,487,263 does not disclose what effects the coagulation using sulphuric acid and gelatin has on storage stability, vulcanization rate and vulcanizate properties, and in particular the modulus, of rubbers and makes no disclosure on the question of the metal corrosivity of corresponding vulcanizates.
In U.S. Pat. No. 4,920,176, it is stated and evidenced by experimental data that very high sodium, potassium and calcium contents and also considerable amounts of emulsifier remain in the nitrile rubber in coagulation of a nitrile rubber latex according to the prior art using inorganic salts such as sodium chloride or calcium chloride. This is undesirable and, according to U.S. Pat. No. 4,920,176, water-soluble cationic polymers are used instead of inorganic salts in the coagulation of nitrile rubber latices for the purpose of obtaining an extremely pure nitrile rubber. The said water-soluble cationic polymers are, for example, ones based on epichlorohydrin and dimethylamine. The vulcanizates obtained therefrom display lower swelling on storage in water and an increased electrical resistance. In the patent text, the property improvements mentioned are attributed purely qualitatively to the minimal cation contents remaining in the product. A more detailed explanation of the phenomena observed is not given.
The objective of EP-A-1 369 436 is to provide nitrile rubbers having a high purity. In order to produce the nitrile rubbers, the emulsion polymerization is carried out in the presence of fatty acid and/or resin acid salts as emulsifiers, then coagulation of the latex is carried out by means of addition of acids with pH values of 6 or less, optionally with addition of precipitants. As acids, it is possible to use all mineral and organic acids which allow the desired pH values to be set. As additional precipitant, it is possible to use, for example, alkali metal salts of inorganic acids. Furthermore, it is mentioned but not demonstrated experimentally that precipitation auxiliaries such as gelatin, polyvinyl alcohol, cellulose, carboxylated cellulose and cationic and anionic polyelectrolytes or mixtures thereof can also be added. The fatty and resin acids formed here are subsequently washed out by means of aqueous alkali metal hydroxide solutions and the polymer is finally subjected to shear until a residual moisture content of less than 20% is obtained. This results in nitrile rubbers having very low residue emulsifier contents and low cation contents (sodium, potassium, magnesium and calcium contents). The chloride contents of the nitrile rubbers described in the two examples are 90 ppm and 111 ppm. EP-A-1 369 436 gives no information on the desired production of nitrile rubbers. In particular, EP-A-1 369 436 does not give any indication of what factors influence the vulcanization rate and the property profile of the associated vulcanizates, in particular the metal corrosivity thereof.
EP-A-0 692 496, EP-A-0 779 301 and EP-A-0 779 300 in each case describe nitrile rubbers based on an unsaturated nitrile and a conjugated diene. All the nitrile rubbers contain 10-60% by weight of unsaturated nitrile and have a Mooney viscosity in the range 15-150 or, according to EP-A-0 692 496, in the range 15-65 and all have at least 0.03 mol of C12-C16-alkylthio group per 100 mol of monomer units, with this alkylthio group having at least three tertiary carbon atoms and a sulphur atom which is bound directly to at least one of the tertiary carbon atoms.
The nitrile rubbers are in each case produced in the presence of a C12-C16-alkyl thiol having a corresponding structure as molecular weight regulator which functions as “chain transfer agent” and is thus incorporated as end group into the polymer chains.
In the case of the nitrile rubbers of EP-A-0 779 300, it is stated that they have a width “ΔAN” (AN=acrylonitrile) of the composition distribution of the unsaturated nitrile in the copolymer in the range from 3 to 20. The process for producing them differs from that of EP-A-0 692 496 in that only 30-80% by weight of the total amount of monomers is used at the beginning of the polymerization and the remaining amount of monomers is fed in only at a conversion of the polymerization of 20-70% by weight.
In the case of the nitrile rubbers of EP-A-0 779 301, it is stated that they contain 3-20% by weight of a fraction having a low molecular weight and a number average molecular weight Mn of less than 35 000. The process for producing them differs from that of EP-A-0 692 496 in that only 10-95% by weight of the alkyl thiol are mixed into the monomer mixture before the polymerization and the remaining amount of the alkyl thiol is fed in only after a polymerization conversion of 20-70% by weight has been reached.
With regard to the coagulation of the latex, all three patent applications EP-A-0 692 496, EP-A-0 779 301 and EP-A-0 779 300 state that any coagulants can be used. As inorganic coagulants, calcium chloride and aluminium chloride are mentioned and used. According to EP-A-0 779 301 and EP-A-0 779 300, one preferred embodiment consists in a nitrile rubber which is substantially halogen-free and is obtained by carrying out latex coagulation in the presence of a nonionic surface-active auxiliary and using halogen-free metal salts such as aluminium sulphate, magnesium sulphate and sodium sulphate. Preference is said to be given to coagulation using aluminium sulphate or magnesium sulphate, in order to obtain the substantially halogen-free nitrile rubber. The nitrile rubber produced in this way in the examples possesses a halogen content of not more than 3 ppm. It is shown that a nitrile rubber of this kind with 3 ppm of chloride yields a vulcanizate with low metal corrosivity. As far as the production of nitrile rubbers with higher chloride contents, and the metal corrosivity of vulcanizates produced from them, are concerned, no statement is made.
According to EP-A-0 692 496, EP-A-0 779 300 and EP-A-0 779 301, it is essential to use alkyl thiols in the form of the compounds 2,2,4,6,6-pentamethylheptane-4-thiol and 2,2,4,6,6,8,8-heptamethylnonane-4-thiol as molecular weight regulators for the production of the nitrile rubbers. It is pointed out that the use of the conventional tert-dodecyl mercaptan as regulator gives nitrile rubbers having poorer properties.

In the case of the nitrile rubbers produced in EP-A-0 692 496, EP-A-0 779 300 and EP-A-0 779 301, it is stated that they have an advantageous property profile, good processability of the rubber mixtures and make low fouling of the mould possible during processing. The vulcanizates obtained are said to have a good combination of low-temperature resistance and oil resistance and possess good mechanical properties. It is also stated that high polymerization conversions of greater than 75%, preferably greater than 80%, in the production of the nitrile rubbers enable a high productivity to be achieved and the vulcanization rate in vulcanization using sulphur or peroxides is high, in particular in the case of NBR grades for injection moulding. It is also indicated that the nitrile rubbers have a short initial vulcanization time and a high crosslinking density. As evidence of the rapid vulcanization of the nitrile rubbers produced according to EP-A-0 692 496, EP-A-0 779 300 and EP-A-0 779 301, the initial vulcanization time (known as the “scorch time” (measured as “Ts”)) is presented, although this is merely a measure of the initial vulcanization rate.
On this subject, EP-A-0 692 496 indicates, inter alia, that many methods have already been proposed for setting high vulcanization rates, e.g. the use of minimal amounts of emulsifiers and precipitants, so that only minimal amounts of emulsifiers and precipitants remain in the NBR.
DE 10 2007 024011 describes a rapidly vulcanizing nitrile rubber having good mechanical properties, in particular a high modulus 300 value, which has an ion index (“II”) according to the general formula (I) in the range from 7 to 26 ppm×mol/g. The ion index is defined as follows:
                              ion          ⁢                                          ⁢          index                =                                            3              ⁢              c              ⁢                                                          ⁢                              (                                  Ca                                      2                    +                                                  )                                                    40              ⁢                                                          ⁢              g              ⁢                              /                            ⁢              mol                                -                      [                                                            c                  ⁡                                      (                                          Na                      +                                        )                                                                    23                  ⁢                                                                          ⁢                  g                  ⁢                                      /                                    ⁢                  mol                                            +                                                c                  ⁡                                      (                                          K                      +                                        )                                                                    39                  ⁢                                                                          ⁢                  g                  ⁢                                      /                                    ⁢                  mol                                                      ]                                              (        I        )            where c(Ca2+*), c(Na+) and c(K+) indicate the concentrations of the calcium, sodium and potassium ions in the nitrile rubber in ppm. The nitrile rubbers produced according to the invention which are mentioned in the examples have Ca ion contents in the range 325-620 ppm and Mg ion contents in the range 14-22 ppm. The nitrile rubbers which are not according to the invention in the examples have Ca ion contents in the range 540-1290 ppm and Mg ion contents of 2-34 ppm. To obtain such a rapidly vulcanizing nitrile rubber, the coagulation is carried out in the presence of a salt of a monovalent metal and optionally a maximum of 5% by weight of a salt of a divalent metal and the temperature during coagulation and subsequent washing is at least 50° C. DE 102007024011 does not contain any details about any possible metal corrosivity of the vulcanizates produced from these nitrile rubbers.
DE 10 2007 024008 describes a particularly storage-stable nitrile rubber which contains 2,2,4,6,6-pentamethylheptane-4-thio and/or 2,4,4,6,6-pentamethylheptane-2-thio and/or 2,3,4,6,6-pentamethylheptane-2-thio and/or 2,3,4,6,6-pentamethylheptane-3-thio end groups and has a calcium ion content of at least 150 ppm, preferably ≧200 ppm based on the nitrite rubber and a chlorine content of at least 40 ppm, based on the nitrile rubber. The Ca ion contents of the nitrile rubbers produced in the examples according to the invention are 171-1930 ppm and the Mg contents are 2-265 ppm. The Ca ion contents of the comparative examples which are not according to the invention are 2-25 ppm, and the Mg ion contents are 225-350 ppm. Such a storage-stable nitrile rubber is obtained when the coagulation of the latex is carried out in the presence of at least one salt based on aluminium, calcium, magnesium, potassium, sodium or lithium and the coagulation or washing is carried out in the presence of a Ca salt or of washing water containing Ca ions and in the presence of a Cl-containing salt. The chlorine contents of the examples according to the invention are situated in the 49 to 970 ppm range, and those of the non-inventive, comparative examples are situated in the 25 to 39 ppm range. The lower chlorine contents, at 25 to 30 ppm, are obtained only, however, when coagulation takes place with chloride-free precipitants such as magnesium sulphate, aluminium sulphate or potassium aluminium sulphate and is followed by washing with deionized water. DE 102007024008 says nothing about the metal corrosivity of these kinds of NBR vulcanizates.
DE 10 2007 024010 describes a further fast-vulcanizing nitrile rubber which has an ion index (“II”) according to the general formula (I) in the range 0-60 ppm×mol/g, preferably 10-25 ppm×mol/g,
                    II        =                              3            ⁡                          [                                                                    c                    ⁡                                          (                                              Ca                                                  2                          +                                                                    )                                                                            40                    ⁢                                                                                  ⁢                    g                    ⁢                                          /                                        ⁢                    mol                                                  +                                                      c                    ⁡                                          (                                              Mg                                                  2                          +                                                                    )                                                                            24                    ⁢                                                                                  ⁢                    g                    ⁢                                          /                                        ⁢                    mol                                                              ]                                -                      [                                                            c                  ⁡                                      (                                          Na                      +                                        )                                                                    23                  ⁢                                                                          ⁢                  g                  ⁢                                      /                                    ⁢                  mol                                            +                                                c                  ⁡                                      (                                          K                      +                                        )                                                                    39                  ⁢                                                                          ⁢                  g                  ⁢                                      /                                    ⁢                  mol                                                      ]                                              (        I        )            where c(Ca2+), c(Mg2+), c(Na+), and c(K+) indicate the concentration of the calcium, magnesium, sodium and potassium ions in the nitrile rubber in ppm, and has an Mg ion content of 50-250 ppm based on the nitrite rubber. In the examples for the nitrile rubbers produced according to the invention, the Ca ion content c(Ca2+) is in the range 163-575 ppm and the Mg ion content c(Mg2+) is in the range 57-64 ppm. In the examples for nitrite rubbers which are not according to the invention, the Ca ion content c(Ca2+) is in the range 345-1290 ppm and the Mg ion content c(Mg2+) is in the range 2-440 ppm. Such nitrile rubbers are obtained if the coagulation of the latex is carried out with adherence to particular measures and the latex is set to a temperature of less than 45° C. before coagulation using a magnesium salt. DE 102007024010 does not contain any details about the chlorine contents of the nitrile rubbers resulting in this process and about the metal corrosivity of vulcanizates produced therefrom.